Electrical Plug

ABSTRACT

A plug is diclosed that is adapted to be inserted into a standard electrical outlet and has a built-in integrated circuit for rectifying the alternating current received from the outlet and limiting the direct current produced. The plug is specifically suited for low-current, low-voltage applications such as supplying DC to a string of ornamental lights. The integrated circuit has a diode with a modified cathode that acts as a fuse as it is rectifying the alternating current. The cathode is formed of a plurality of electrically parallel conductors. Excessive current blows out at least some of these conductors in the same way a conventional fuse blows. Various embodiments add other current-limiting elements, elements for producing full wave or near full wave rectification, and for opening the circuit when they detect extraneous currents.

BACKGROUND OF THE INVENTION

1. Field of the Invention:

The present invention relates to small wattage electrical power needssuch as ornamental light strings for decorating trees. In particular,the present invention relates to integrated circuits for rectifying andlimiting current from an AC source.

2. Discussion of Background:

Strings of individual, small wattage lights are widely used to decorateChristmas trees. Although these types of decorations are enjoyed by manypeople, they are sometimes a fire hazard, resulting in tragedies everyChristmas holiday season. Most of these light strings draw a current ofat least 200 mA at 120 VAC. Because of the concern over safety, theselight strings may be made with heavier gauge wire, 22 gauge rather than24.

In addition to Christmas tree light strings, there are otherapplications for electrical power for devices that have low power needsand especially those that use direct current (DC) rather thanalternating current (AC), such as many devices that use integratedcircuits, such as calculators or devices that use light-emitting diodes.AC or large voltage surges can ruin these types of devices.

There have been some attempts to make Christmas tree decorations safer.Many of these involve putting fire or smoke detectors in ornaments. Anexample of this can be found in U.S. Pat. No. 4,075,614 issued to Whiteand in U.S. Pat. No. 5,396,221 issued to Bridges. Fuses have beenincorporated into the plugs of light strings, as taught for example byCheng in U.S. Pat. Nos. 4,227,228 and Lehmann et al in 3,968,398.However, none of these address all of the problems with these types oflight strings.

Smith et al, in U.S. Pat. No. 4,675,575, describe a light string basedon lightemitting diodes (LEDs) which can be driven by pulsed DC power,which in turn is powered by household AC voltage. The pulsed DC isproduced by a silicon controlled rectifier from the AC input.

DC is not unknown in connection with Christmas tree lighting using moreconventional type incandescent bulbs. See Wu's stages selection typeChristmas tree light controller circuit in U.S. Pat. No. 5,345,147, andKramer's decorative lighting system in U.S. Pat. No. 3,789,211. However,in both cases the DC power is used only tangentially in achieving aparticular lighting effect and not to power the bulbs themselves.

Turning briefly to the figures, FIGS. 1A, 1B and 1C illustrate graphs ofvoltages versus time, with voltage represented by the verticalcoordinate and time represented by the horizontal coordinate. FIG. 1Aillustrates an alternating current (AC), meaning that the currentreverses the direction of flow. So-called household current variesbetween approximately ±60 volts and does so at approximately 60 timesper second. Direct current (DC) does not change direction, although itmay change amplitude. FIGS. 1B and 1C both illustrate DC; FIG. 1B showshalf-wave DC and Fig. 1C shows full wave DC. Both also illustrate pulsedDC.

It is known to generate half-wave and full wave DC from AC by devicescalled converters or rectifiers. Devices for rectifying AC to DC aresometimes based on diodes, which are solid state circuit elements thatconduct current in the forward direction but not in the reversedirection. For half-wave rectification, as illustrated in FIG. 1B, asingle diode can exclude negative voltages, leaving only positivevoltages. For full-wave rectification, two or four diodes are used.

Some of the patents identified above employ diodes in various functionsincluding in relation to power sources, such as disclosed by Smith etal. However, there remains a need for a better way to power low wattagedevices and Christmas tree light strings in particular.

SUMMARY OF THE INVENTION

According to its preferred embodiments and briefly described, thepresent invention is a plug, attachable to an electrical outlet andcontaining an integrated circuit designed to yield a DC output to alow-current, low-voltage load such as, for example, a light string thatmight be used for decorating a Christmas tree. Within the plug, thenovel and non-obvious elements and combinations of elements providesignificant and, in some embodiments, redundant protection from currentsurges and current leakage while reducing energy consumption. Moreover,the resulting light is whiter than typical incandescent bulbsilluminated by AC.

A major advantage of the present invention is that it operates on a muchlower current, typically one fifth the amperage of conventional lightstrings. This advantage results in reduced operating temperature forwhich translates into significant cost savings when the vast number ofthese types of light strings in use is considered. Notwithstanding thelower current requirements, the quality of light is improved and appearsto be whiter light.

Another important advantage of the present invention is that thesimplicity of the circuits translates into low manufacturing costs, infact, significantly lower costs than conventional strings with heaviergauge wire.

Still other features of the present invention are certain circuitcomponents that are included for safety reasons, for example, a specialdiode, to be described more fully herein, but which, simply put,operates like a fuse. Also, the present circuit includes special currentsensing devices that open the circuit to shut down the lights if anover-current condition should arise from some internal and/or extraneousload condition, and diodes that prevent AC from getting to the lights.

Other features and their advantages will be apparent to those skilled inlow wattage power supplies from a careful reading of the DetailedDescription of Preferred Embodiments accompanied by the followingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the figures,

FIGS. 1A, 1B and 1C are graphs illustrating, respectively, alternatingcurrent, half-wave pulsed direct current and full-wave pulsed directcurrent versus time;

FIG. 2 is a circuit diagram according to a first preferred embodiment ofthe present invention;

FIG. 3 is a circuit diagram according to a second, alternative preferredembodiment of the present invention;

FIGS. 4A and 4B are circuit diagrams according to third and fourthalternative preferred embodiments of the present invention, which arevariations of the first and second embodiments, respectively;

FIG. 5 is a circuit diagram illustrating yet a fifth alternativeembodiment of the present invention, which is a variation of the thirdembodiment of FIG. 4A;

FIG. 6 is a circuit diagram illustrating a circuit for use with any ofthe first through fifth embodiments of FIGS. 2-5;

FIG. 7 is an exterior front perspective view of a plug for a wall socketcontaining a circuit, according to a preferred embodiment of the presentinvention; and

FIG. 8 is a rear perspective view of the plug of FIG. 7 with a portionof the housing of the plug cut away to show an integrated circuitinside, according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention can be viewed from several standpoints. First, thepresent invention is circuitry for use in a plug housing adapted to beattached to an electrical outlet by inserting its contacts into theholes of an outlet and to provide power for a low wattage, low voltagedevice such as a series of Christmas lights. It will be clear to thosefamiliar with these types of devices that other applications, especiallythose that use integrated circuits, can employ the present circuitry insuch a plug.

The present invention is also a "string of lights", which means aplurality of incandescent light bulbs, each in one of a series ofsockets adapted to apply an electric potential across the filament inthe bulb so as to cause it to glow from resistance heating, each lightbeing connected by electrically conducting wires so that it iselectrically in series and/or in parallel with the other, and having theimproved plug at one end of the string for insertion into a wall socketthat delivers household current, which plug is in electrical connectionwith the electrically conducting wires and thus each of the light bulbs.

One of several preferred, alternative embodiments is illustrated in FIG.2. Inputs are generally indicated by reference numerals 10 and 12;outputs of an integrated circuit 18 are indicated by reference numbers20 and 22. Fuses 14 and 16 may be installed in-line between inputs 10and 12, respectively, and integrated circuit 18, and, if so, arepreferably rated at no more than approximately 180 milliamperes each.The output of integrated circuit 18 is half-wave, 54 volts rectified DCfor 120 volts AC input and is fed to a load such as a light string 28.The components shown inside integrated circuit 18 are preferablymanufactured as a unit.

Inside integrated circuit 18 are two diodes 24 and 26. Diode 24 acts asa half-wave rectifier and a fuse, and is referred to herein as a solidstate rectifying fuse (SSRF) diode, or "Gibboney" diode, after theinventor. The N junction of this diode is modified from that ofconventional diodes to limit current. If the design current for diode 24is exceeded, say, going past 100 milliamperes during the positivecurrent flow portion of the cycle, diode 24 will open in a mannersimilar to a fuse. The modification to diode 24 comprises forming aplurality of small conducting strips at the cathode that are sufficientin combination to carry the rated current. These strips are preferablyon the order of one to ten microns in width. Diode 24 can bemanufactured with a nominal rating of one or more amperes, but byburning out or severing some of the strips, its rating can be adjusteddownward from the nominal rating for specific applications. In thissense, it is a programmable diode. For Christmas tree light strings, anamperage rating of 150 milliamps may be suitable.

Diode 26, a conventional diode rated at 100 times the peak half cyclesurge current of fuses 14 and 16, shunts the current from input 12 tooutput 20 to prevent AC from getting to the lights or other loads ifintegrated circuit 18 shorts out. Diodes 24 and 26 obviate the need forfuses 14 and 16, especially fuse 14, but fuses 14 and 16 are shown inFIGS. 2-6 because they are required under current industry standards.Alternatively, if diode 24 is replaced by a conventional diode, fuse 14and possibly also fuse 16 may still be needed.

FIG. 3 illustrates an alternative embodiment of the present invention.As with the embodiment shown in FIG. 2, there are inputs 10 and 12,outputs 30 and 32, fuses 14 and 16 and integrated circuit 40. There isalso SSRF diode 24 and resistors 34, 36 and 38. The output of circuit 40is full-wave rectification; however, the amplitude of the secondhalf-wave is reduced compared to that of the first half-wave in order tolimit current. Circuit 40 uses the negative half of the cycle to fill inbetween positive halves with a lower voltage pulse to keep the lampfilaments from cooling down. To achieve this result, the voltage of thepositive half of the cycle passes through SSRF diode 24 and then tooutput 30. During the negative half of the cycle, current flows acrossresistor 36 and then across resistor 34, but after the first of thesetwo voltage drops, part of the current is tapped for output 32. Thelevel of the output voltage from the negative half of the input cyclewill depend on the ratio of resistors 34 to 36, and the magnitude ofthese resistors determines the amount of current that can be sourced bythe load. This embodiment, especially when connected to a householdcurrent system having a 50 cycle frequency instead of 60, would avoidthe shimmering effect that the embodiment of FIG. 2, or any otherhalf-wave embodiment, would inevitably produce.

FIGS. 4A and 4B illustrate integrated circuits 18 and 40 modified aswill be described below. However, components common to FIGS. 2 and 3will have the same reference numbers to simplify the understanding ofthe drawings. In FIG. 4A, a circuit 50 is in line between inputs 10 and12 and outputs 20 and 22. Diodes 24 and 26 have the same functions incircuit 50 as they do in circuit 18. In addition to diodes 24 and 26,however, is a conventional thermistor 52, or other current-limitingdevices, to vary the resistance with the temperature or voltage,generally increasing in resistance as its temperature rises. Here, untilthe rated current of thermistor 52 is exceeded, its resistance is verylow. Once that current is exceeded, however, the temperature ofthermistor 52 rises significantly as does its resistance, to the orderof megaohms, essentially opening the circuit. Once current isinterrupted, thermistor 52 cools down in a second or two.

FIG. 4B illustrates an integrated circuit 60 that parallels circuit 40of FIG. 3, including inputs 10 and 12, outputs 30 and 32, diode 24,resistors 34, 36, and 38. Circuit 60 includes thermistor 52 for the samepurposes as described with respect to circuit 50.

FIG. 5 illustrates yet another embodiment of the present invention.Between inputs 10 and 12, fuses 14 and 16, and outputs 20 and 22, is anintegrated circuit 70 containing SSRF diodes 24 and diode 26, as in theembodiments of FIGS. 2 and 4. In addition, integrated circuit 70 hasthermistor 52 of embodiments 4A and 4B. These elements have the samefunctions as previously described. Integrated circuit 70 also has anoptional full wave diode bridge 72 composed of four standard diodes 74to feed full wave DC through SSRF diode 24. Diode 26 is not needed ifbridge 72 is used.

Further, integrated circuit 70 has a programmable voltage array 76, astandard circuit element, in line so that the output voltage can bedetermined by the user, preferably by specifying that voltage to themanufacturer. Array 76 is a voltage divider or a chopper circuit,depending on the current demands of the application and is well known inthe art. For a Christmas tree light string, a chopper circuit ispreferred, and its frequency would be less than 10 kilohertz.

All of the foregoing integrated circuits, 18, 40, 50, 60 and 70 may beused in combination with an extraneous current load interruptor 80 whichcan sense a current leakage out of the loop by looking at the currentsupplied by the source and demanded by the load. Interruptor 80's inputs82 and 84 couple directly to outputs 20 and 22 of each of the integratedcircuits. The voltage drop of the current flowing into each input 82, 84of interruptor 80 is sampled using small, well-matched resistors 86 and88, preferably on the order of one milliohm each, by operationalamplifiers 90 and 92. The outputs of operational amplifiers 90 and 92are input to a summing amplifier 94. As long as the currents in bothinputs are the same, then the voltage of the outputs of operationalamplifiers 90 and 92 at summing amplifier 94 are the same and its outputis zero. If there is a difference, on the order of one or two milliampsor less, then the output of summing amplifier 94 goes to saturation andcauses a control logic element 96 to produce an"on"output that opens asolid state relay or switching device such, as CMOS 98, to interruptpower in the circuit. Because the control logic element 96 samples theoutput of summing amplifier 94 at at least 1 megahertz frequency,shutting down after detecting extranous currents takes about six closckcycles or less than one millisecond. To reset, the plug incorporatinginterrupter 80 must be pulled from the wall socket and then reinserted.Interruptor 80 can be used with DC or AC.

These integrated circuits, with or without extraneous current loadinterrupter, can be used with various small lights commonly found inconventional AC light strings, such as "minis", C7s and C9s, as well asother small lights used now in only DC applications, including theso-called "grain of rice" lights that run from 1/8th inches to 3/16thinches long and 1/16th inches in diameter.

FIGS. 7 and 8 illustrate a front and back perspective view of a plugaccording to the present invention that is adapted for insertion into aconventional two- or three-hole electrical outlet (not shown). Plug 100includes a non-conducting housing 110 from which two electrical contacts112 and 114 extend. Contacts 112 and 114 fit into two of the holes ofthe outlet. As is known in the art, contact 114 is slightly wider thancontact 112 so that plug 100, when inserted in the electrical outlet orsocket, is always inserted with the same orientation to assure that thepolarity is correct.

From housing 110 also extends a pair of electrically conducting,insulated wires 116, 118, for conducting electricity to a load 124. Load124 may be a string of lights or other low voltage, low current device,such as a calculator or LED array.

Inside housing 110 are fuses 120 and 122 which correspond to fuses 14and 16, respectively, of integrated circuit 18 of FIG. 2 describedabove. Wire 118 runs to fuse 122; wire 116 runs to integrated circuit130 and then to fuse 116. Integrated circuit 130 can comprise integratedcircuit 18, 40, 50, 60 or 70, either alone or in combination withinterrupter 80. For simplicity, support structure for the internalcomponents of housing 110, and preferably molded integrally with it, hasbeen eliminated in FIG. 8.

It will be clear to those skilled in the art of integrated circuits andlow voltage power supplies that many modifications and substitutions canbe made to the foregoing preferred embodiments without departing fromthe spirit and scope of the invention, which is defined by the followingclaims.

What is claimed is:
 1. A plug adapted to be connected to an electricaloutlet and to receive alternating current, said plug being adapted to bepart of an electrical circuit that delivers direct current to anelectrical load, said plug comprising:a housing; a first contact and anopposing second contact, said first and said second contacts carried bysaid housing and extending therefrom so as to be insertable into theelectrical outlet to connect said plug with said outlet; a pair ofelectrically conducting wires running into said housing; and a solidstate rectifying fuse diode, carried by said housing in electricalconnection with said first and said second contacts and said pair ofelectrically conducting wires, for rectifying and limiting saidalternating current from said first and second contacts so that onlydirect current flows to said pair of wires, and if a preselected currentlevel is reached, said diode will open said circuit and thereby stop itfrom conducting any current to said load.
 2. The plug as recited inclaim 1, further comprising means connected between said diode and saidwires for detecting differences between electrical currents in saidwires, when said wires are attached to said electrical load and saiddiode.
 3. The plug as recited in claim 2, further comprising meansresponsive to said detecting means for halting the flow of directcurrent to the load when a difference is detected by said detectingmeans.
 4. The plug as recited in claim 1, further comprising a shuntdiode for preventing alternating current from being conducted to theload.
 5. The plug as recited in claim 1, further comprising means forlimiting current to said diode.
 6. The plug as recited in claim 5,wherein said limiting means is selected from the group consisting of athermistor and fuse.
 7. The plug as recited in claim 1, furthercomprising a full-wave rectifying bridge connected between said wiresand said diode.
 8. The plug as recited in claim 1, wherein said diodeproduces a half-wave in each cycle of said alternating current, saidhalf wave having a first amplitude, and further comprising means inelectrical connection between said first and second contacts and saiddiode for producing a second half wave in each of said cycles, saidsecond half-wave having a second amplitude equal to or lower than saidfirst amplitude.
 9. A light string, comprising:a plurality of lightbulbs; a plurality of light sockets, each light socket of said pluralityof light sockets adapted to receive one light bulb of said plurality oflight bulbs; electrical connectors interconnecting said each socket; aplug in electrical connection to said electrical connectors, said plughavinga housing, a pair of contacts carried by said housing and adaptedto be inserted into an electrical socket, an integrated circuit carriedby said housing in electrical connection with said electrical connectorsand said pair of contacts, and a diode in said integrated circuit forrectifying and limiting alternating current so that direct current isconducted to said bulbs through said electrical connectors when saidcontacts are inserted into the electrical socket.
 10. The light stringas recited in claim 9, wherein said diode produces a half-wave in eachcycle of said alternating current, said half wave having a firstamplitude, and further comprising means in electrical connection betweensaid first and second contacts and said diode for producing a secondhalf-wave in each of said cycles, said second half-wave having a secondamplitude lower than said first amplitude.
 11. The light string asrecited in claim 9, further comprising a current-limiting device carriedby said integrated circuit between said contacts and said diode forlimiting current to said diode.
 12. The light string as recited in claim9, further comprising a thermistor carried by said integrated circuitbetween said contacts and said diode for limiting current to said diode.13. The light string as recited in claim 9, further comprising arectifying bridge carried by said integrated circuit between saidcontacts and said diode.
 14. The light string as recited in claim 9,further comprising:means connected between said diode and saidelectrical connectors for detecting differences between electricalcurrents in said electrical connectors when said wires are attached tosaid electrical load and said diode; and means responsive to saiddetecting means for halting the flow of direct current to the load whena difference is detected by said detecting means.
 15. The light stringas recited in claim 14, wherein said diode produces a half-wave in eachcycle of said alternating current, said halfwave having a firstamplitude, and further comprising:means in electrical connection betweensaid first and second contacts and said diode for producing a secondhalf-wave in each of said cycles, said second half-wave having a secondamplitude lower than said first amplitude; and a current-limiting devicecarried by said integrated circuit between said contacts and said diodefor limiting current to said diode.
 16. An electrical circuit elementfor use with an electrical circuit, said circuit element comprising asolid state rectifying fuse diode having an anode and a cathode, saidcathode comprising a plurality of conductors arranged to be electricallyin parallel with each other and in series with said anode so that, ifelectrical current to said diode exceeds a preselected limit, at leastsome conductors of said plurality of conductors open to limit currentpassed by said conductor.
 17. The electrical circuit element as recitedin claim 16, further comprising means for opening some of saidconductors to change the preselected limit of said diode.
 18. Anintegrated circuit element for use with an electrical circuit having asource side and a drain side, said source side having a first electricalcurrent, and said drain side having a second electrical current, saidintegrated circuit element comprising:means for detecting a differencebetween said first and said second electrical currents; means responsiveto said detecting means for opening said electrical circuit if saiddifference is detected.
 19. The integrated circuit element as recited inclaim 18, wherein said detecting means further comprises:a summingamplifier having two inputs; two operational amplifiers, eachoperational amplifier having two inputs and one output, said output ofsaid operational amplifier forming into one input of said two inputs ofsaid summing amplifier, said inputs measuring an electrical potentialdrop across a small resistance of an electrical circuit, one operationalamplifier measuring a voltage change across a resistance in said sourceside, and the other operational amplifier measuring a voltage changeacross a resistance in said drain side, said summing amplifier comparingsaid electrical potentials of said source and said drain sides.
 20. Theintegrated circuit as recited in claim 18, wherein said detecting meansfurther comprises:logic means responsive to said detecting means fordetermining whether a difference has been detected; and relay meansresponsive to said logic means for opening said electrical circuit whensaid logic means detects said difference.